The present invention relates generally to networks and, more particularly, to admission control in networks.
Data experiences latency and jitter (variances in latency) as it travels through a network from a source to a destination. Latency may be caused by several characteristics of the network, such as the communications medium the data is transported over, processing of the data as it travels through the network, congestion in the network, and other factors. Some causes of latency are unavoidable, such as physical limits of transport media. An example of a physical limit is the speed of light in an optical transport medium. Depending on the application, some latency and jitter may be acceptable. For example, for some applications it may be acceptable to have a certain average latency, latency variance, or some combination of these. At some point, however, long latency and high jitter become unacceptable for most applications.
FIG. 1 illustrates the relationship between offered traffic load and latency in a network. As the offered load of traffic to the network increases, latency also increases. This general relationship between latency and offered load is a fundamental property of networks. At some point the amount of latency becomes unacceptable, which is generally defined as the point of congestion. Congestion is denoted in FIG. 1 by the horizontal line labeled “congestion.” Below the congestion line congestion does not occur, which is characterized by traffic that has acceptable latency and/or jitter characteristics. Above the line, congestion occurs. Congestion is characterized by traffic having unacceptably high latency, jitter, or both. When there is almost no load, latency drops to the lowest delay the network is capable of.
Examples of latency-sensitive applications include voice, video, and real-time applications that need data to reach a destination within a certain time period. For latency-sensitive applications, there are negative consequences if latency and jitter between a source and a destination is above a certain threshold. Voice data, for example, may suffer from discernible audio glitches at the destination. Similarly, if video data is subjected to high latency and jitter the visual quality of the video data will be degraded at the destination. Therefore, these types of data are often transmitted over circuit-switched networks that maintain required latency guarantees for the data, thus eliminating degradation caused by latency and jitter at the destination. For other applications, such as e-mail and file transfers, high latencies (within limits) may cause little or no discernible degradation at the destination.
Connectionless networks, such as the Internet, do not guarantee bandwidth. Thus, networks entirely or partially made up of connectionless networks are not set up to guarantee maximum latency levels from end to end. Rather, in a network that is entirely or partially a connectionless network, latency and jitter increase when load on the network increases and decrease when the load on the network decreases. Furthermore, latency and jitter may increase in certain parts of a network and decrease in other parts. Therefore, it is difficult to convey latency- and jitter-sensitive data in such networks. Without a mechanism to specifically control latency and jitter, networks that are entirely or partially connectionless are not appropriate for carrying latency- or jitter-sensitive data.